TheDowd–Beckwith ring-expansion reaction is anorganic reaction in which a cyclic carbonyl (typically a β-ketoester) is expanded by up to 4 carbons in afree radicalring expansion reaction through an α-alkylhalosubstituent.^[1][2][3] The radical initiator system is based onazobisisobutyronitrile andtributyltin hydride.^[1] The cyclic β-keto ester can be obtained through aDieckmann condensation. The original reaction consisted of anucleophilic aliphatic substitution of theenolate of ethyl cyclohexanone-2-carboxylate with 1,4-diiodobutane andsodium hydride followed by ring expansion to ethyl cyclodecanone-6-carboxylate. A side-reaction isorganic reduction of the iodoalkane.

Thereaction mechanism involves a bicyclic intermediate. The reaction is initiated by thermal decomposition ofAIBN. The resultingradicals abstract hydrogen fromtributyltin hydride to a tributyltin radical which in turn abstracts thehalogen atom to form analkyl radical. This radical attacks thecarbonyl group to an intermediatebicyclicketyl. This intermediate thenrearranges with ring expansion to a new carbon radical species which recombines with a proton radical from tributyltin hydride propagating thecatalytic cycle.

A side reaction accompanying this ring expansion isorganic reduction of the halo alkane to a saturated alkyl group. One study^[4] shows that the success depends critically on the accessibility of the carbonyl group.Deuterium experiments also show the presence of a 1,5 hydride shift. The reaction of the alkyl radical with the ester carbonyl group is also a possibility but has an unfavorableactivation energy.

• ^A new tributyltin hydride-based rearrangement of bromomethyl .beta.-keto esters. A synthetically useful ring expansion to .gamma.-keto esters Paul Dowd, Soo Chang Choi;J. Am. Chem. Soc.;1987; 109(11); 3493–3494.doi:10.1021/ja00245a069
• ^Free radical ring expansion by three and four carbons Paul Dowd, Soo Chang Choi;J. Am. Chem. Soc.;1987; 109(21); 6548–6549.doi:10.1021/ja00255a071
• ^Rearrangement of suitably constituted aryl, alkyl, or vinyl radicals by acyl or cyano group migration Athelstan L. J. Beckwith, D. M. O'Shea, Steven W. Westwood; J. Am. Chem. Soc.;1988; 110(8); 2565–2575.doi:10.1021/ja00216a033
• ^Three-Carbon Dowd–Beckwith Ring Expansion Reaction versus Intramolecular 1,5-Hydrogen Transfer Reaction: A Theoretical Study Diego Ardura and Tomás L. SordoJ. Org. Chem.;2005; 70(23) pp. 9417–9423; (Article)doi:10.1021/jo051551g

• Rearrangement reactions
• Name reactions
• Ring expansion reactions

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